This disclosure generally relates to systems and processes for capturing carbon dioxide (CO2) from gas streams, and more particularly to a solids delivery system to handle solid material containing the captured CO2.
The emission of carbon dioxide into the atmosphere from industrial sources such as power plants is now considered to be a principal cause of the “greenhouse effect”, which contributes to global warming. In response, efforts are underway to reduce emissions of CO2. Many different processes have been developed to attempt to accomplish this task. Examples include polymer and inorganic membrane permeation; removal of CO2 by adsorbents such as molecular sieves; cryogenic separation; and scrubbing with a solvent that is chemically reactive with CO2, or which has a physical affinity for the gas.
One technique has received much attention for removing CO2 from flue gas streams, e.g., exhaust gas produced at power plants. In this technique, aqueous monoethanolamine (MEA) or hindered amines like methyldiethanolamine (MDEA) and 2-amino-2-methyl-1-propanol (AMP) are employed as the solvents in an absorption/stripping type of regenerative process. The technique has been demonstrated commercially, using pilot plant and/or slipstream units, for CO2 capture from coal-fired power plants and gas turbines. Commercial CO2 capture has been practiced in gas sweetening processes for chemical production and in the food and beverage industry.
There are certainly considerable advantages inherent in the MEA and hindered amine-based absorption processes. However, a number of deficiencies may be preventing wider adoption of this type of technology. For example, the process can sometimes result in sharp increases in the viscosity of the liquid sorbent, which can cause decrease the mass transfer of CO2 into the sorbent. To avoid this problem, the concentration of MEA and other amines is sometimes maintained at a relatively low level, e.g., below about 30 wt % in water, in the case of MEA. However, the lower concentrations can greatly reduce absorbing capacity, as compared to the theoretical capacity of the neat absorbent.
Moreover, energy consumption in the MEA process can be quite high, due in large part to the need for solvent (e.g., water) heating and evaporation. For example, the process may consume about 10-30% of the steam generated in a boiler that is heated by combustion of a fossil fuel. Furthermore, MEA-based absorption systems may not have the long-term thermal stability, in the presence of oxygen, in environments where regeneration temperatures typically reach at least about 120° C.
Additional drawbacks may result from the fact that the liquid sorbent which is enriched with CO2 in the MEA or hindered amine process may still contain a substantial amount of free amine and solvent (usually water). The amine and water can be evaporated under typical operating conditions, and can cause corrosion and other degradation in the attendant equipment. To address this concern, specialized, corrosion-resistant materials can be used for the equipment, but this can in turn increase capital costs for the plant. In some cases, corrosion inhibitors can be added, but the use of these specialized additives can also increase operational costs.
Another example of a commercial CO2 post-combustion capture process uses aqueous solutions of piperazine-promoted potassium carbonate (K2CO3). However, this process is often very energy-intensive, and can be economically inferior to the MEA process. Still another example involves the use of chilled ammonia. In this case, energy-intensive cooling systems are usually required for such a system, and the risks associated with unintended ammonia release may be unacceptable.
Therefore, there remains a need for systems that efficiently and effectively remove carbon dioxide from a gaseous stream.